Magnetic Tunnel Junction Device

ABSTRACT

The output voltage of an MRAM is increased by means of an Fe(001)/MgO(001)/Fe(001) MTJ device, which is formed by microfabrication of a sample prepared as follows: A single-crystalline MgO (001) substrate is prepared. An epitaxial Fe(001) lower electrode (a first electrode) is grown on a MgO(001) seed layer at room temperature, followed by annealing under ultrahigh vacuum. A MgO(001) barrier layer is epitaxially formed on the Fe(001) lower electrode (the first electrode) at room temperature, using a MgO electron-beam evaporation. A Fe(001) upper electrode (a second electrode) is then formed on the MgO(001) barrier layer at room temperature. This is successively followed by the deposition of a Co layer on the Fe(001) upper electrode (the second electrode). The Co layer is provided so as to increase the coercive force of the upper electrode in order to realize an antiparallel magnetization alignment.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a Continuation of U.S. application Ser. No.13/400,340 filed on Feb. 20, 2012, which is a Continuation of U.S.application Ser. No. 12/923,643 filed on Sep. 30, 2010, which is aContinuation of U.S. application Ser. No. 10/591,947 filed on Sep. 8,2006, which is a National Stage Application of PCT/JP2005/004720 filedon Mar. 10, 2005. Priority is claimed from U.S. application Ser. No.13/400,340 filed on Feb. 20, 2012, which claims priority from U.S.application Ser. No. 12/923,643 filed on Sep. 30, 2010, which claimspriority from U.S. application Ser. No. 10/591,947 filed on Sep. 8,2006, which claims priority from PCT/JP2005/004720 filed on Mar. 10,2005, which claims priority from Japanese Application Nos. 2004-313350and 2004-071186 filed Oct. 28, 2004 and Mar. 12, 2004, respectively, thecontent of which is hereby incorporated by reference into thisapplication.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a magnetic tunnel junction device and amethod of manufacturing the same, particularly to a magnetic tunneljunction device with a high magnetoresistance and a method ofmanufacturing the same.

2. Description of Related Art

Magnetoresistive random access memories (MRAMs) refer to a large-scaleintegrated memory circuit that is expected to replace the currentlywidely used DRAM memories. Research and development of MRAM devices,which are fast and non-volatile memory devices, are being extensivelycarried out, and sample products of a 4 Mbit MRAM have actually beendelivered.

FIGS. 8(A) and 8(B) show the structure and operation principle of amagnetic tunnel junction device (to be hereafter referred to as a “MTJdevice”), which is the most important part of the MRAM. As shown in FIG.8(A), a MTJ device comprises a tunnelling junction structure in which atunnel barrier (to be hereafter also referred to as a “barrier layer”)made of an oxide is sandwiched between a first and a second electrodemade of a ferromagnetic metal. The tunnel barrier layer comprises anamorphous Al—O layer (see Non-Patent Document 1). As shown in FIG. 8(A),in the case of parallel magnetization alignment where the directions ofmagnetizations of the first and second ferromagnetic electrodes arealigned parallel, the electric resistance of the device with respect tothe direction normal to the interfaces of the tunneling junctionstructure decreases. On the other hand, in the case of antiparallelmagnetization alignment where the directions of magnetizations of thefirst and second ferromagnetic electrodes are aligned antiparallel asshown in FIG. 8(B), the electric resistance with respect to thedirection normal to the interfaces of the tunneling junction structureincreases. The resistance value does not change in a general state, sothat information “1” or “0” can be stored depending on whether theresistance value is high or not. Since the parallel and antiparallelmagnetization alignments can be stored in a non-volatile fashion, thedevice can be used as a non-volatile memory device.

FIG. 9 shows an example of the basic structure of the MRAM. FIG. 9(A)shows a perspective view of the MRAM, and FIG. 9(B) schematically showsa circuit block diagram. FIG. 9(C) is a cross-section of an example ofthe structure of the MRAM. Referring to FIG. 9(A), in an MRAM, a wordline WL and a bit line BL are disposed in an intersecting manner, withan MRAM cell disposed at each intersection. As shown in FIG. 9(B), theMRAM cell disposed at the intersection of a word line and a bit linecomprises a MTJ device and a MOSFET directly connected to the MTJdevice. Stored information can be read by reading the resistance valueof the MTJ device that functions as a load resistance, using the MOSFET.The stored information can be rewritten by applying a magnetic field tothe MTJ device, for example. As shown in FIG. 9(C), an MRAM memory cellcomprises a MOSFET 100 including a source region 105 and a drain region103 both formed inside a p-type Si substrate 101, and a gate electrode111 formed on a channel region that is defined between the source anddrain regions. The MRAM also comprises a MTJ device 117. The sourceregion 105 is grounded, and the drain is connected to a bit line BL viathe MTJ device. A word line WL is connected to the gate electrode 111 ina region that is not shown.

Thus, a single non-volatile MRAM memory cell can be formed by a singleMOSFET 100 and a single MTJ device 117. The MRAMs are therefore suitablewhere high levels of integration are required.

Non-Patent Document 1: D. Wang, et al.: Science 294 (2001) 1488.

SUMMARY OF THE INVENTION

Although there are prospects for achieving MRAMs with capacities on theorder of 64 Mbits based on the current technologies, the characteristicsof the MTJ device, which is the heart of MRAM, needs to be improved ifhigher levels of integration are to be achieved. In particular, in orderto increase the output voltage of the MTJ device, the magnetoresistancemust be increased and the bias voltage characteristics must be improved.FIG. 10 illustrates how the magnetoresistance in a conventional MTJdevice using an amorphous Al—O as the tunnel barrier changes as afunction of the bias voltage (L1). As shown, in the conventional MTJdevice, the magnetoresistance is small and, notably, it tends todrastically decrease upon application of bias voltage. With suchcharacteristics, the output voltage when operation margins are takeninto consideration is too small for the device to be employed for anactual memory device. Specifically, the magnetoresistance of the currentMTJ device is small at approximately 70%, and the output voltage is alsosmall at no more than 200 mV, which is substantially half the outputvoltage of a DRAM. This has resulted in the problem that as the level ofintegration increases, signals are increasingly lost in noise and cannotbe read.

It is an object of the invention to increase the output voltage of a MTJdevice. It is another object of the invention to provide a memory devicewith a high magnetoresistance for stable operation.

In one aspect, the invention provides a magnetoresistive devicecomprising a magnetic tunnel junction structure comprising: a tunnelbarrier layer; a first ferromagnetic material layer of the BCC structureformed on a first side of the tunnel barrier layer; and a secondferromagnetic material layer of the BCC structure formed on a secondside of the tunnel barrier layer, wherein the tunnel barrier layer isformed by a single-crystal MgO_(x) (001) layer or a poly-crystallineMgO_(x) (0<x<1) layer in which the (001) crystal plane is preferentiallyoriented.

The invention further provides a magnetoresistive device comprising amagnetic tunnel junction structure comprising: a tunnel barrier layercomprising MgO(001); a first ferromagnetic material layer comprisingFe(001) formed on a first side of the tunnel barrier layer; and a secondferromagnetic material layer comprising Fe(001) formed on a second sideof the tunnel barrier layer, wherein the MgO layer is formed by asingle-crystalline MgO_(x) (001) layer or a poly-crystalline MgO_(x)(0<x<1) layer in which the (001) crystal plane is preferentiallyoriented. In a preferred embodiment, the band discontinuity value (theheight of the tunnel barrier) between the bottom of the conduction bandof the MgO(001) layer and the Fermi energy of the Fe(001) layer issmaller than an ideal value of a perfect single-crystal without defect.These features increase the magnetoresistance and thereby allow theoutput voltage of the MTJ device to be increased. By using any of theaforementioned MTJ devices as a load for a single transistor, anon-volatile memory can be formed.

In another aspect, the invention provides a method of manufacturing amagnetoresistive device comprising: preparing a substrate; depositing afirst Fe(001) layer on the substrate; depositing a tunnel barrier layeron the first Fe(001) layer by electron beam evaporation under highvacuum, the tunnel barrier layer comprising a single-crystalline MgO_(x)(001) or a poly-crystalline MgO_(x) (0<x<1) in which the (001) crystalplane is preferentially oriented; and forming a second Fe(001) layer onthe tunnel barrier layer.

The invention furthermore provides a method of manufacturing a MTJdevice comprising a first step of preparing a substrate comprising asingle-crystalline MgO_(x)(001) or a poly-crystalline MgO_(x) (0<x<1) inwhich the (001) crystal plane is preferentially oriented, a second stepof depositing a first Fe(001) layer on the substrate and performing anannealing process to make the surface flat, a third step of depositing atunnel barrier layer on the first Fe(001) layer by electron beamevaporation, the tunnel barrier layer comprising a single-crystallineMgO_(x)(001) or a poly-crystalline MgO_(x) (0<x<1) in which the (001)crystal plane is preferentially oriented, and a fourth step of forming asecond Fe(001) layer on the tunnel barrier layer. The method may furthercomprise the step of growing a seed layer between the first and thesecond steps, the seed layer comprising a single-crystallineMgO_(x)(001) or a poly-crystalline MgO_(x) (0<x<1) in which the (001)crystal plane is preferentially oriented. The MgO layer may be depositedusing a target with the value of x in MgO_(x) adjusted. The value of xin MgO_(x) may be adjusted in the step of forming the MgO.

In yet another aspect, the invention provides a magnetoresistive devicecomprising a magnetic tunnel junction structure comprising a tunnelbarrier layer comprising MgO(001), a first ferromagnetic material layercomprising an amorphous magnetic alloy formed on a first side of thetunnel barrier layer, and a second ferromagnetic material layercomprising an amorphous magnetic alloy formed on a second side of thetunnel barrier layer, wherein the discontinuous value (the height of thetunnel barrier) between the bottom of the conduction band of theMgO(001) layer and the Fermi energy of the first or the secondferromagnetic material layer comprising the amorphous magnetic alloy islower than an ideal value of a perfect single-crystal with no defect.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1(B) shows the structure of a MTJ device according to a firstembodiment of the invention, and FIG. 1(A) shows the energy bandstructure of a ferromagnetic metal Fe(001), illustrating the E-E_(F)relationship with respect to the [001] direction of the momentum space.

FIG. 2(A) to FIG. 2(D) schematically show the process of manufacturing aMTJ device with a Fe(001)/MgO(001)/Fe(001) structure (to be hereafterreferred to as a “Fe(001)/MgO(001)/Fe(001) MTJ device”) according to anembodiment of the invention.

FIG. 3(A) shows a RHEED image of a Fe(001) lower electrode (a firstelectrode), and FIG. 3(B) shows a RHEED image of a MgO(001) barrierlayer.

FIG. 4 shows the results of observing the quadrupole mass spectra in thedeposition chamber during the MgO evaporation.

FIG. 5 shows the film deposition rate dependency of the oxygen partialpressure during the MgO evaporation.

FIG. 6 shows typical magnetoresistance curves of theFe(001)/MgO(001)/Fe(001) MTJ device.

FIG. 7(A) shows the bias voltage dependency of the MR ratio at roomtemperature, and FIG. 7(B) shows the output voltage V_(out) of the MTJdevice (=bias voltage×(Rap−Rp)/Rap).

FIGS. 8(A) and 8(B) show the structure of the MTJ device and itsoperating principle.

FIG. 9 shows an example of the basic structure of an MRAM, FIG. 9(A)showing a perspective view of the MRAM, FIG. 9(B) showing a schematiccircuit diagram, and FIG. 9(C) showing a cross-sectional view of anexample of its structure.

FIG. 10 shows how the magnetoresistance of a conventional MTJ deviceusing an amorphous Al—O as the tunnel barrier changes depending on thebias voltage.

FIG. 11 shows the structure of a MTJ device according to a variation ofthe invention, corresponding to FIG. 1(B).

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the context of the present specification, because MgO has a cubiccrystal structure (NaCl structure), the (001) plane, the (100) plane,and the (010) plane are all equivalent. The direction perpendicular tothe film surface is herein considered to be the z-axis so that the filmplane can be uniformly described as (001). Also in the context of thepresent specification, BCC structure, which the crystalline structure offerromagnetic electrode layer, means body-centered cubic latticestructure. More specifically, BCC structure includes the BCC structurewith no chemical ordering so-called A2-type structure, the BCC structurewith chemical ordering such as B2-type structure and L2 ₁-typestructure, and also the aforementioned structures with slight latticedistortion.

The term “ideal value” with regard to a perfect single-crystal withoutdefect herein refers to a value that has been estimated from ultravioletphotoemission spectroscopy experiments (see W. Wulfhekel, et al.: Appl.Phys. Lett. 78 (2001) 509.). The term “ideal value” is used hereinbecause the aforementioned state can be considered to be an upper limitvalue of the potential barrier height of the tunnel barrier of an idealsingle-crystal MgO with hardly any oxygen vacancy defect or latticedefect.

Before describing the preferred embodiments of the invention, ananalysis conducted by the inventors is discussed. The magnetoresistance(MR) ratio of a MTJ device can be expressed by the following equation:

ΔR/R _(p)=(Rap−Rp)/Rp

where Rp and Rap indicate the tunnel junction resistance in the cases ofparallel and antiparallel magnetization alignments, respectively, of twoelectrodes. According to the Jullire's formula, the MR ratio at low biasvoltage can be expressed by:

MR ratio=(Rap−Rp)/Rp=2P ₁ P ₂/(1−P ₁ P ₂), and

Pa=(Dα↑(E _(F))−Dα↓(E _(F)))/(Dα↑(E _(F))+Dα↓(E _(F)),  (1)

where α=1, 2

In the above equations, Pα is the spin polarization of an electrode, andDα ↑ (E_(F)) and Dα ↓ (E_(F)) are the density of state (DOS) at theFermi energy (E_(F)) of the majority-spin band and the minority-spinband, respectively. Since the spin polarization of ferromagnetictransition metals and alloys is approximately 0.5 or smaller, theJullire's formula predicts a highest estimated MR ratio of about 70%.

Although the MR ratio of approximately 70% has been obtained at roomtemperature when a MTJ device was made using an amorphous Al—O tunnelbarrier and polycrystalline electrodes, it has been difficult to obtainthe output voltage of 200 mV, which is comparable to the output voltagesof DRAMs, thereby preventing the realization of MRAM as mentioned above.

The inventors tried an approach to deposit a MTJ device in which thetunnel barrier comprises a single-crystal (001) of magnesium oxide (MgO)or a poly-crystalline MgO in which the (001) crystal plane ispreferentially oriented. It is the inventors' theory that, becausemagnesium oxide is a crystal (where the atoms are arranged in an orderlyfashion), as opposed to the conventional amorphous Al—O barrier, theelectrons are not scattered and the coherent states of electrons areconcerved during the tunneling process. FIG. 1(B) shows the MTJ devicestructure according to an embodiment of the invention. FIG. 1(A) showsthe energy band structure of the ferromagnetic Fe(001), that is, theE-E_(F) relationship with respect to the [001] direction of the momentumspace. As shown in FIG. 1(B), the MTJ device structure of the presentembodiment comprises a first Fe (001) layer 1, a second Fe (001) layer5, and a single-crystalline MgO_(x) (001) layer 3 or a poly-crystallineMgO_(x) (0<x<1) layer 3 sandwiched therebetween, the polycrystallinelayer having the (001) crystal plane preferentially oriented therein.According to the aforementioned Jullire's model, assuming that themomentum of the conduction electrons is preserved in the tunnelingprocess, the tunneling current that passes through MgO would bedominated by those electrons with wave vector k_(z) in the directionperpendicular to the tunnel barrier (normal to the junction interfaces).In accordance with the energy band diagram shown in FIG. 1(A) of Fe inthe [001] (Γ-H) direction, the density of state (DOS) at the Fermienergy E_(F) does not exhibit a very high spin polarization due to thefact that the sub-bands of the majority-spin and the minority-spin havestates at the Fermi energy E_(F). However, in case the coherent statesof electrons are conserved in the tunneling process, only thoseconduction electrons that have totally symmetrical wave functions withrespect to the axis perpendicular to the barrier would be coupled withthe states in the barrier region and come to have a finite tunnelingprobability. The Δ1 band in the Fe(001) electrode has such totallysymmetric wave functions. As shown in FIG. 1(A), the majority spin Δ1band (solid line) has states at the Fermi energy E_(F), whereas theminority spin Δ1 band (broken line) does not have state at the Fermienergy E_(F). Because of such half-metallic characteristics of the Fe Δ₁band, there is the possibility that a very high MR ratio can be obtainedin a coherent spin polarized tunneling. Since in an epitaxial(single-crystal, or (001) oriented poly-crystal) MTJ device thescattering of electrons is suppressed during the tunneling process, anepitaxial MTJ device is thought to be ideal for realizing theaforementioned coherent tunneling.

In the following, a MTJ device according to a first embodiment of theinvention and a method of manufacturing the same will be described withreference to the drawings. FIGS. 2(A) to 2(D) schematically show themethod of manufacturing the MTJ device having the Fe(001)/MgO(001)/Fe(001) structure according to the embodiment (to behereafter referred to as a “Fe(001)/MgO(001)/Fe(001) MTJ device”).Fe(001) refers to a ferromagnetic material with the BCC structure.First, a single-crystal MgO(001) substrate 11 was prepared. In order toimprove the morphology of the surface of the single-crystal MgO(001)substrate 11, a MgO(001) seed layer 15 was grown by the molecular beamepitaxy (MBE) method. This was subsequently followed by the growth of anepitaxial Fe(001) lower electrode (first electrode) 17 with thethickness of 50 nm on the MgO(001) seed layer 15 at room temperature, asshown in FIG. 1(B), and then annealing was performed at 350° C. underultrahigh vacuum (2×10⁻⁸ Pa). Electron-beam evaporation conditionsincluded an acceleration voltage of 8 kV, a growth rate of 0.02 nm/sec,and the growth temperature of room temperature (about 293K). The sourcematerial of the electron-beam evaporation was MgO of the stoichiometriccomposition (the ratio of Mg to 0 being 1:1), the distance between thesource and the substrate was 40 cm, the base vacuum pressure was 1×10⁻⁸Pa, and the O₂ partial pressure was 1×10⁻⁶ Pa. Alternatively, a sourcewith oxygen vacancy defects may be used instead of the MgO of thestoichiometric composition (the ratio of Mg to O is 1:1).

FIG. 3(A) shows a RHEED image of the Fe(001) lower electrode (a firstelectrode). The image shows that the Fe(001) lower electrode (firstelectrode) 17 possesses a good crystallinity and flatness. Thereafter, aMgO(001) barrier layer 21 with the thickness of 2 nm was epitaxiallygrown on the Fe(001) lower electrode (first electrode) at roomtemperature, also using the MgO electron-beam evaporation method. FIG.3(B) shows a RHEED image of the MgO(001) barrier layer 21. The imageshows that the MgO(001) barrier layer 21 also possesses a goodcrystallinity and flatness.

As shown in FIG. 2(D), a Fe(001) upper electrode (a second electrode) 23with the thickness of 10 nm was formed on the MgO(001) barrier layer 21at room temperature. This was successively followed by the deposition ofa Co layer 25 with the thickness of 10 nm on the Fe(001) upper electrode(second electrode) 23. The Co layer 25 is provided to increase thecoercive force of the upper electrode 23 so as to realize theantiparallel magnetization alignment. The thus prepared sample was thenprocessed by microfabrication techniques to obtain theFe(001)/MgO(001)/Fe(001) MTJ device.

The aforementioned MgO evaporation using an electron beam involved theformation of a film under ultrahigh vacuum of 10⁻⁹ Torr. It can be seenthat in this method, the film, even when formed on a glass substrate tothe thickness of 300 nm, was colorless and transparent, showing that agood crystal film was formed. FIG. 4 shows the results of observing thequadrupole mass spectra in the deposition chamber during the MgO growth.

The results show that the partial pressures regarding the spectrum P1 ofO and the spectrum P2 of O₂ are high. FIG. 5 shows the film depositionrate dependency of the oxygen partial pressure during MgO evaporation.It will be seen from the figure that the oxygen partial pressure itselfis high, and that the oxygen partial pressure increases as thedeposition rate increases. These results indicate the separation ofoxygen from MgO during the deposition of MgO. Since the separated oxygenis pumped out of the deposition chamber using vacuum pumps, there is thepossibility that there are oxygen vacancy defects such as MgO_(x)(0.9<x<1). When there are oxygen vacancy defects, the potential barrierheight of the MgO tunnel barrier is thought to decrease (such as in therange of 0.10 to 0.85 eV; more specifically, 0.2 to 0.5 eV), which isthought to result in an increase in the tunneling current. In the caseof a typical Al—O tunnel barrier, the height of the tunnel barrier withrespect to Fe(001) electrodes is considered to be 0.7 to 2.5 eV. Anideal tunnel barrier height of a MgO crystal is 3.6 eV, and experimentalvalues of 0.9 to 3.7 eV have been obtained. Using the method accordingto the present embodiment of the invention, a tunnel barrier height ofapproximately 0.3 eV is expected, indicating that the resistance of thetunnel barrier can be lowered. However, it should be noted that otherfactors, such as the influence of the aforementioned coherent tunneling,might also be involved. The value of x in MgO_(x) due to oxygen vacancydefects is such that 0.98<x<1, and more preferably 0.99<x<1. These arethe ranges such that the sole presence of Mg is excluded and thecharacteristics of MgO can be basically maintained.

The aforementioned tunnel barrier height φ was determined by fitting theelectric conductance characteristics of the MTJ device (the relationshipbetween tunnel current density J and bias voltage V) onto the Simmons'formula (Equation (20) in a non-patent document by J. G. Simmons: J.Appl. Phys. 34, pp. 1793-1803 (1963)) based on the WKB approximation,using the least squares method. The fitting was performed using the freeelectron mass (m=9.11×10⁻³¹ kg) as the electron's effective mass. When abias voltage V (which is normally on the order of 500 mV to 1000 mV) isapplied until non-linearity appears in the J-V characteristics, theheight φ of the tunnel barrier and the effective thickness Δs of thetunnel barrier can be simultaneously determined by fitting the J-Vcharacteristics using the Simmons' formula.

The effective thickness Δs of the tunnel barrier was determined to besmaller than the thickness of the actual MgO(001) tunnel barrier layer(t_(MgO)) determined from a cross-sectional transmission electronmicroscope image of the MTJ device by approximately 0.5 nm. This is theresult of the effective thickness Δs of the tunnel barrier having beenreduced from the actual MgO(001) layer thickness by the effect of theimage potential produced at the interface between the MgO(001) layer andthe alloy layer consisting mainly of Fe and Co.

It is noted that, in the event that t_(MgO) can be accurately determinedusing the cross-sectional transmission electron microscope (TEM) image,the height φ of the tunnel barrier can be more simply determined by thefollowing technique. Namely, when the bias voltage V applied to the MTJdevice is small (normally 100 mV or smaller), the tunnel current densityJ is proportional to the bias voltage V, such that the J-Vcharacteristics become linear. In such a low-bias voltage region, theSimmons' formula can be described as follows:

J=[(2mφ)^(1/2) /Δs](e/h)²×exp[−(4πΔs/h)×(2mφ)^(1/2) ]×V  (2)

where m is the mass of the free electron (9.11×10⁻³¹ kg), e is theelementary electric charge (1.60×10⁻¹⁹ C), and h is the Planck'sconstant (6.63×10⁻³⁴ J·s). The effective thickness of the tunnel barrierΔs is approximately t_(MgO)−0.5 nm. By fitting the J-V characteristicsof the MTJ device in the low-bias voltage region onto Equation (2), theheight φ of the tunnel barrier can be simply and yet accuratelyestimated.

FIG. 6 shows a typical magnetoresistance curve of theFe(001)/MgO(001)/Fe(001) MTJ device produced by the above-describedmethod. The MR ratio is 146% at the measurement temperature of 20K and88% at the measurement temperature of 293K. These values represent thehighest MR ratios that have so far been obtained at room temperature.Such high MR ratios cannot be explained by the spin polarization of theFe(001) electrode and is thought rather to be related to a coherentspin-polarized tunneling. When 160 prototype MTJ devices were made, thevariations regarding the MR ratio and tunneling resistance value werenot more than 20%. The yield of the MTJ devices was 90% or more at thelaboratory stage. These high values suggest the effectiveness of theapproach of the invention.

The resistance-area (RA) product of the MTJ device was on the order of afew KΩμm², which is suitable for MRAM.

FIG. 7( a) shows the bias voltage dependency of the MR ratio at roomtemperature. It will be seen that the bias voltage dependency of the MRratio is fairly small. Although the characteristics are asymmetric, thevoltage V_(half) at which the MR ratio is reduced in half of thezero-bias value is 1250 mV, which is a very high value. In thisconnection, it is noted that the voltage V_(half) at which the MR ratiois reduced in half of the zero-bias value in the conventional MTJs withAl—O tunnel barrier is 300 to 600 mV. FIG. 7( b) shows the outputvoltage V_(out) of the MTJ device (=bias voltage×(Rap−Rp)/Rap). Themaximum value of the output voltage V_(out) is 380 mV with a positivebias. This value is about twice as large as that (a little less than 200mV) in the case of the Al—O barrier. These high values in terms of bothMR ratio and output voltage suggest the effectiveness of the techniqueaccording to the present embodiment.

Although in the above-described embodiment Fe(001) of BCC was employed,an Fe alloy of BCC, such as an Fe—Co alloy, Fe—Ni alloy, or Fe—Pt alloy,may be used instead. Alternatively, a layer of Co or Ni with thethickness of one or several monoatomic layers may be inserted betweenthe electrode layer and the MgO(001) layer.

Hereafter, a MTJ device according to a second embodiment of theinvention and a method of manufacturing the same will be described. Inthe method of manufacturing a Fe(001)/MgO(001)/Fe(001) MTJ deviceaccording to the present embodiment, MgO(001) is initially deposited ina poly-crystalline or amorphous state by sputtering or the like, andthen an annealing process is performed such that a poly-crystal in whichthe (001) crystal plane is preferentially oriented or a single-crystalis obtained. The sputtering conditions were such that, for example, thetemperature was room temperature (293K), a 2-inch φ MgO was used as atarget, and sputtering was conducted in an Ar atmosphere. Theacceleration power was 200 W and the growth rate was 0.008 nm/s. BecauseMgO that is deposited under these conditions is in an amorphous state, acrystallized MgO was obtained by increasing the temperature to 300° C.from room temperature and maintaining that temperature for a certainduration of time.

Oxygen vacancy defects may be introduced by a method whereby oxygenvacancy defects is produced during growth, a method whereby oxygenvacancy defects is introduced subsequently, or a method whereby a statewith oxygen vacancy defects is subjected to an oxygen plasma process ornatural oxidation so as to achieve a certain oxygen deficit level.

As described above, in accordance with the MTJ device technology of thepresent embodiment, an annealing process is carried out forcrystallization after an amorphous MgO has been deposited by sputtering,thereby eliminating the need for large-sized equipment.

Hereafter, a MTJ device according to a variation of the embodiments ofthe invention will be described with reference to the drawings. FIG. 11shows the structure of the MTJ device according to the variation, whichcorresponds to FIG. 1(B). As shown in FIG. 11, the MTJ device of thevariation is characterized in that, as in the MTJ device of theabove-described embodiments, the electrodes disposed on either side of asingle-crystal MgO_(x)(001) layer 503 or an oxygen-deficit poly-crystalMgO_(x)(0<x<1) in which the (001) crystal plane is preferentiallyoriented comprises an amorphous ferromagnetic alloy, such as CoFeBlayers 501 and 505. The amorphous ferromagnetic alloy can be formed byevaporation or sputtering, for example. The resultant characteristicsare substantially identical to those of the first embodiment.

As the amorphous magnetic alloy, FeCoB, FeCoBSi, FeCoBP, FeZr, and CoZrmay be used, for example. Although an anneal process after thepreparation of the MTJ device might cause the amorphous magnetic alloyin the electrode layers to be partially or entirely crystallized, thiswould not lead to a significant deterioration of the MR ratio. Thus,such a crystallized amorphous magnetic alloy may be used in theelectrode layers.

While the MTJ device according to various embodiments of the inventionhas been described, it should be apparent to those skilled in the artthat the invention is not limited to those specific embodiments andvarious other modifications, improvements and combinations are possible.For example, the height of the tunnel barrier may be adjusted by dopingCa or Sr, instead of introducing an oxygen vacancy defects to the MgOlayer. Further, while the MgO layer has been described to be depositedby electron-beam evaporation or sputtering, it should be obvious thatother deposition methods are also possible. The term “high vacuum”refers to values on the order of no more than 10⁻⁶ Pa in the case whereoxygen is not introduced, for example. In the case where oxygen isintroduced, the term refers to values on the order of 10⁻⁴ Pa.

In accordance with the invention, a larger magnetoresistance than in theconventional MTJ device can be obtained, and the output voltage of theMTJ device can be increased. At the same time, the resistance value ofthe MTJ device can be reduced so that it is optimized for MRAM. Theinvention thus enables the level of integration of MRAM using the MTJdevice to be readily increased. In accordance with the invention, theoutput voltage value of the MRAM roughly doubles over prior art, makingthe MTJ device of the invention suitable for very large scale integratedMRAMs of gigabit class.

What is claimed is:
 1. A tunnel barrier layer located between a first ferromagnetic material layer and a second ferromagnetic material layer, wherein the tunnel barrier layer comprises a poly-crystalline magnesium oxide layer in which a (001) crystal plane is preferentially oriented.
 2. The tunnel barrier layer according to claim 1, wherein the tunnel barrier layer has a barrier height φ in a range of 0.2 to 0.5 electron volts (eV).
 3. The tunnel barrier layer according to claim 2, wherein the tunnel barrier height φ is obtained by fitting J-V characteristics of a tunnel barrier junction structure to an equation (1): J=[(2mφ)^(1/2) /Δs](e/h)²×exp[−(4πΔs/h)×(2mφ)^(1/2) ]×V  (1) where J is a tunnel current density flowing through the tunnel barrier layer, V is an applied bias voltage that is 100 mV or smaller, m is the free electron mass, e is the elementary electric charge, h is the Planck's constant, Δs is an effective thickness of the tunnel barrier layer that is approximately equivalent to (t_(MgO)−0.5 nm), and t_(MgO) is an actual thickness of the tunnel barrier layer determined using a cross-sectional transmission electron microscope image.
 4. A magnetoresistive device having a tunnel barrier junction structure, the magnetoresistive device comprising: a first ferromagnetic material layer; a second ferromagnetic material layer; and a tunnel barrier layer located between the first and second ferromagnetic material layers, and wherein the tunnel barrier layer comprises a single-crystalline magnesium oxide layer in which a (001) crystal plane is preferentially oriented or a poly-crystalline magnesium oxide layer in which a (001) crystal plane is preferentially oriented, and wherein the tunnel barrier layer has a tunnel barrier height φ in a range of 0.2 to 0.5 eV.
 5. The magnetoresistive device according to claim 4, wherein at least one of the first and second ferromagnetic material layers comprises CoFeB alloy.
 6. The magnetoresistive device according to claim 4, wherein the tunnel barrier height φ is obtained by fitting J-V characteristics of the tunnel barrier junction structure to an equation (1); J=[(2mφ)^(1/2) /Δs](e/h)²×exp[−(4πΔs/h)×(2mφ)^(1/2) ]×V  (1) where J is a tunnel current density flowing through the tunnel barrier layer, V is an applied bias voltage that is 100 mV or smaller, m is the free electron mass, e is the elementary electric charge, h is the Planck's constant, Δs is an effective thickness of the tunnel barrier layer that is approximately equivalent to (t_(MgO)−0.5 nm), and t_(MgO) is an actual thickness of the tunnel barrier layer determined using a cross-sectional transmission electron microscope image.
 7. The magnetoresistive device according to claim 6, wherein at least one of the first and second ferromagnetic material layers comprises CoFeB alloy.
 8. A magnetoresistive device having a tunnel barrier junction structure, the magnetoresistive device comprising: a first ferromagnetic material layer deposited on a substrate; a second ferromagnetic material layer; and a tunnel barrier layer located between the first and second ferromagnetic material layers, and wherein the tunnel barrier layer comprises a single-crystalline magnesium oxide in which a (001) crystal plane is preferentially oriented or a poly-crystalline magnesium oxide in which a (001) crystal plane is preferentially oriented, and wherein at least the first ferromagnetic material layer comprises an iron-based alloy and is crystallized.
 9. The magnotoresistive device according to claim 8, wherein the second ferromagnetic material layer comprises an iron-based alloy and is crystallized.
 10. A magnetoresistive device having a tunnel barrier junction structure, the magnetoresistive device comprising: a first ferromagnetic material layer deposited on a substrate; a second ferromagnetic material layer; and a tunnel barrier layer located between the first and second ferromagnetic material layers, and wherein the tunnel barrier layer comprises a single-crystalline magnesium oxide in which a (001) crystal plane is preferentially oriented or a poly-crystalline magnesium oxide in which a (001) crystal plane is preferentially oriented, and wherein at least the first ferromagnetic material layer comprises an iron-based alloy and is entirely crystallized.
 11. The magnetoresistive device according to claim 10, wherein the second ferromagnetic material layer comprises an iron-based alloy and is entirely crystallized. 